1. Field of the Invention
The present invention relates to a method, system, and program for establishing and using a point-in-time copy relationship.
2. Description of the Related Art
Computing systems often include one or more host computers (“hosts”) for processing data and running application programs, direct access storage devices (DASDs) for storing data, and a storage controller for controlling the transfer of data between the hosts and the DASD. Storage controllers, also referred to as control units or storage directors, manage access to a storage space comprised of numerous hard disk drives, such as a Direct Access Storage Device (DASD), Redundant Array of Independent Disks (RAID), Just a Bunch of Disks (JBOD), etc. Hosts may communicate Input/Output (I/O) requests to the storage space through the storage controller.
In many systems, data on one storage device, such as a DASD, may be copied to the same or another storage device so that access to data volumes can be provided from two different devices. A point-in-time copy involves physically copying all the data from source volumes to target volumes so that the target volume has a copy of the data as of a point-in-time. A point-in-time copy can also be made by logically making a copy of the data and then only copying data over when necessary, in effect deferring the physical copying. This logical copy operation is performed to minimize the time during which the target and source volumes are inaccessible.
One such logical copy operation is known as FlashCopy® (FlashCopy is a registered trademark of International Business Machines, Corp. or “IBM”). FlashCopy® involves establishing a logical point-in-time relationship between source and target volumes. A bitmap is maintained for each volume having a bit value for each track indicating whether the data for a track is located in the volume or another volume. During the establishment operation, all the bit values in the bitmap for the target volume are set to “on” to indicate that the data for the target volume is at another location. After the relationship is established, if an attempt is made to stage data to a track, the bit value in the bitmap for the volume including the track is checked. If the bit value is “on”, indicating the track is a target track subject to a point-in-time copy relationship, then the target track is staged from the corresponding source track in the source device into the target cache. If an access attempts to destage a source track from source cache, the bitmap for the corresponding target volume having the target track is checked. If the bit value in the target bitmap is “one”, then the target track is staged from the source device to the target cache, then destaged from the target cache to the target device and the bit value in the target bitmap for the destaged track from target cache is set to “off”. The access to the source track is only granted after the target relationship bitmap indicates that there is no longer any need to stage the source track to the target cache.
Prior art operations to tracks subject to a point-in-time copy relationship, such as a FlashCopy relationship, require that the target bitmap be readable to both the source and target volumes so that stages and destages can be coordinated to maintain the point-in-time copy relationship. To allow the target bitmap to be readable to both the source and target volumes, in the prior art, the source and target volumes involved in a point-in-time copy relationship must be in the same cluster and logical subsystem. A cluster comprises a separate processor complex, cache, and power boundary, so that if one cluster in a system fails, as part of a failover, the surviving cluster can handle I/O requests. A logical subsystem (LSS) is a logical structure used for configuration of the device. A logical subsystem comprises logical functions of a storage controller that allow one or more host I/O interfaces to access a set of devices. One or more logical subsystems may exist on a storage controller and a given set of devices is typically associated with only one logical subsystem.
There is a need in the art to provide improved techniques for implementing and using a point-in-time copy relationship.